The c-ets-2 proto-oncogene encodes a phosphoprotein of approximately 56-64 Kd and is localized in the nucleus. The ets- 2 mRNA has been shown to be induced in regenerating mouse liver, suggesting that the ets-2 gene expression may be involved in cell proliferation. In order to test the expression of ets-2 on growth properties of mouse fibroblasts, we have constructed expression vectors capable of producing ets-2 protein in NIH 3T3 cells. The mouse and human ets-2 cDNAs were cloned into an expression vector carrying an inducible promoter. The NIH 3T3 cells transfected with the construct carrying mouse c-ets-2 genes showed discrete foci of densely-growing cells when maintained in low-serum or serum-free medium. These transfected cells formed colonies in soft agar and also induced tumors in nude mice. The control cells did not grow in serum-free medium and in the presence of low-serum they did not show characteristic foci observed with ets-2-transfected cells. Our results suggest that the ets-2 proto-oncogene expression induces growth and morphological alterations in NIH 3T3 cells, and abolishes their serum requirement and is tumorgenic in nude mice. These properties provide an assay system to study the functions of ets-2 and its related genes in cell proliferation.